ON TWO SPECIES OF SPHEX. 77 



As I had always found the number of spiders in each cell 

 unequal, but apparently regulated by tbeir size, I opened a 

 range of cells of the sphex Pennsylvanica, and having weigh- 

 ing tbe contents I found the result as follows. See No. III. 

 The lowest contained 19 spiders and a small worm, grains. 

 which seemed lately hatched, and had eaten nothing. 

 See Fig. I. — The spiders weighed . . 74. 



The next contained 17 spiders and one empty skin, 



the worm, Fig. 2, weighed 4 of a grain, the spiders 6L 

 The third contained 19 very small spiders and a i'cw 



empty skins, weighing . .51. 



The worm, Fig. 3, weighed 4 



The fourth contained only ihe empty skins of the spi- 

 ders. The worm, Fig. 4, seemed lean and weak, 

 he was just beginning to spin. I think he must have 

 had a short allowance provided for him, or have 

 been sick: he weighed . . . . 3L 



The fifth contained an involucrum in which was a large 

 grub not yet changed to a chrysalis. The involu- 

 crum and worm being heavier than the last, weigh- 

 ed . . . . . . 34. 



The 6th and 7th cells were open at the point, the young 

 sphex having escaped. 



This examination proves that the sphex exercises a nice 

 degree of judgment in the quantity of provision he lays up. 

 For the cell No. 3. must have contained 22 or 23 spiders, 

 and I have often counted only 6 or 7 in one, but they were 

 of a large size. It also appears that the full-grown worm 

 weighs about half as much as the food that reared him. 



If it be further necessary to break the line which has for- 

 merly been drawn between reason and instinct, the economy 

 of the whole class of hymenoptera, and especially of the 

 sphex, will contribute towards it, I will relate a singular in- 

 stance of conduct in which instinct appears to be out of the 

 question. 



In order to examine one of these insects (the Pennsj lvanica) 

 at work, I raised a picture frame a little from the wall. In 

 doing this, I injured several of his cells, for the dirt sticking 



